1. Field of the Invention
The invention relates generally to wind turbines, and more particularly to systems and methods for positioning wind turbines at the edges of building roofs in order to utilize concentrated wind flow at the edge of the roof for purposes such as electricity generation.
2. Related Art
Wind turbines of many types have been used to generate electricity in a variety of ways. The prior art describes turbines in a variety of locations including natural ridgelines, open fields, mesas and on ships. Recent design concepts for rebuilding the World Trade Center showed plans to incorporate wind turbines in that structure to harness the electrical generating power of passing winds. These turbines for generating electricity take many forms including the following:
Horizontal Axis Turbines (traditional-looking windmills) operate with fan-like rotors that look like airplane propellers that face into or away from the wind. The Horizontal Axis Turbine is the most common turbine configuration used today.
The Darrieus Turbine which looks roughly cylindrical, with wind-catching blades that span the length of the cylinder often described as looking like an egg beater. This vertical axis turbine has vertical blades that rotate into and out of the wind, the blades are placed roughly parallel in an arc to the axis. Guy cables are usually used to keep the turbine erect. In addition to keeping the turbine erect, the cables impose a large thrust loading on the main turbine bearings, causing increased wear on the bearings. With this type of turbine, replacing main bearings requires that the turbine be taken down. The Darrieus was invented in the 1920's and is not widely used today.
The Darrieus Turbine is also related to machines with straight vertical axis blades called Giromills or cycloturbines which use a wind vane to mechanically orient a blade pitch change mechanism. They were designed to be mounted on a tower or other device. The cylcoturbine was marketed commercially for several years, but never progressed beyond the research stage.
The Savonius Turbine is S-shaped if viewed from above. The turbine turns relatively slowly, but yields high torque. It has been proven useful for pumping water and other tasks, but its slow rotational speeds have not been proven to generate electricity cost effectively from wind power.
The present types of turbines in commercial use are generally deployed in open fields or mesas, as far from wind-slowing obstructions as possible to allow wind to strike their airfoils or blades at maximum speed. They are raised above the ground as much as possible to bring the airfoils away from wind-slowing ground effects.
Augmentors or concentrators have also been developed to try to concentrate wind onto the turbines, but they have not been proven to be economically successful in commercial use.